This invention relates to distance-measuring interferometry, and in particular, to mirrors for reflecting measurement beams in an interferometric system.
In the fabrication of an integrated circuit, there are steps during which a wafer rests on a moveable stage under a projector. As the stage moves, it experiences linear translation in a direction parallel to one of two coordinate axes that define the plane of the wafer. During these steps, it is desirable to know, with great precision, the position of the moveable stage relative to the projector.
One approach to determining the motion of the stage is to use an interferometer to determine the distance to a measurement spot along the edge of the stage with a measurement beam of an interferometer. A mirror attached to the stage at the measurement spot reflects the measurement beam back to the interferometer. By combining this returning beam with a reference beam, the interferometer detects motion of the mirror, and hence the stage.
As the stage translates, the measurement spot moves relative to the mirror, which, being attached to the stage, also translates with the stage. As a result, the laser beam, which illuminates the measurement spot, illuminates different portions of the mirror as the stage translates in a direction transverse to the direction of beam propagation. To the extent that the mirror is perfectly flat, the fact that the laser beam illuminates different portions of the mirror makes no difference to the measurement of distance.
In many cases, the mirror will deform. Such deformation can arise, for example, from internal stresses within the stage to which the mirror is attached or integrated with. These deformations affect the profile of the mirror""s reflective surface, causing it to be bowed or twisted in unpredictable ways.
Deformations resulting from internal stresses in the mirror body can, to some extent, be reduced by making the mirror body sufficiently massive. However, the additional mass of such a mirror body, when attached to the stage, make it more difficult to accelerate the stage. This reduces the throughput of the system.
In a strain-isolated mirror according to the invention, stresses experienced by the body of the mirror are prevented from being communicated to the reflecting surface of the mirror. As a result, even if the body of the mirror deforms, the reflecting surface remains flat. This is achieved by carving two portions, a body portion and a blade portion, out of a substrate and providing a coupling between these two portions that isolates the blade portion from stresses experienced by the body portion.
In one embodiment, a substrate having first and second body portions forms the strain-isolated mirror. First and second connecting portions extend across a gap between the first body portion and the second body portion. A first flexural hinge on the second connecting portion couples the second connecting portion to the first body portion.
In one aspect, the second connecting portion can also include a second flexural hinge for coupling the second body portion to the second connecting portion. In another aspect, the first connecting portion also includes a third flexural hinge for coupling the first body portion to the first connecting portion.
The second connecting portion can include a central portion and a first end portion adjacent to the first body portion. In this case, the central portion is thicker than the first end portion. The second connecting portion can also include a second end portion adjacent to the second body portion. In this case, the central portion is again thicker than the second end portion also.
The first connecting portion can also include first and second end portions separated by a central portion. The first and second connecting portions are both thicker than the central portion. This results in the formation of a structure having a waist.
In another embodiment, the strain-isolated mirror includes first and second body portions and a support system to connect the first and second body portions to each other while isolating the second body portion from stress in the first body portion.
These and other features and advantages of the invention will be apparent from the following detailed description and the accompanying figures, in which: